Heat transfer system



Jan. 17, 1956 Filed Oct. 25, 1954 N. E. M LEAN HEAT TRANSFER SYSTEM 2 Sheets-Sheet 1 IN V EN TOR.

NORMA/V E. MAOLEAN ATTORNEYS Jan. 17, 1956 N. E. M cLEAN HEAT TRANSFER SYSTEM 2 Sheets-Sheet 2 Filed Oct. 25, 1954 INVENTOR. NORMA/V E. MAOLEA/V 7' TOR/VEYS United States Patent HEAT TRANSFER SYSTEM Norman E. MacLean, San Francisco, Calif.

Application October 25, 1954, Serial No. 464,219

. 3 Claims. (Cl. 62-125) This invention relates to the refrigeration of space such as the interior of railway cars, trucks, ships holds or other space provided for the storage or transportation of perishable merchandise.

The system disclosed herein is generaly similar to that shown in my co-pending application entitled Refrigerating Unit, filed May 6, 1954, Serial No. 427,924, now abandoned. In said co-pending application, Dry Ice is utilized as a primary refrigerant and a highly voltatile substance is employed in its liquid and gaseous phases in a sealed circulating system as a secondary refrigerant. In such a system the heat required for the sublimation of the Dry Ice is obtained from the circulating secondary refrigerant which absorbs the heat of the space to be cooled.

It is the object of the present invention to enhance and make more efiicient the operation of a system such as that disclosed in my co-pending application by utilizing a very small quantity of power derived outside the system.

A further object of the invention is to augment the operation of a thermo-siphonic action in a refrigerating system by introducing a small motor-actuated pump in such a manner that it will tend to evacuate or cause low pressure on the evaporator side of the system While at the same time slightly increasing the pressure onthe condenser side of the system.

Further and more specific objects and advantages of the invention are made apparent in the following specification whereinreference is made to the accompanying drawings for a detailed description thereof.

In the drawings:

Fig. 1 is a schematic view in vertical section of a refrigerating system embodying the present invention, and

Fig. 2 is a sectional view of a portion of the system taken on the line IIII of Fig. 1.

In the drawings, the space to be refrigerated is illustrated in part as enclosed by insulated walls of hollow construction preferably filled with a material pervious to gas such as rock wool, glass fiber or other insulating substance. The refrigerating unit disposed within this space is partially contained in a sealed enclosure 11 provided with an access door, not shown, through which blocks of Dry Ice such as indicated at 12 may be inserted. These blocks of Dry Ice rest upon shelves 13 and 14 and act as a primary refrigerant for a secondary refrigerant which circulates in a sealed system presently to be described.

The sealed system for the secondary refrigerant comprises a cooling unit which includes tubes 15 and headers 16; the tubes passing in close proximity to the ice and being inclined to encourage ready flow of the secondary refrigerant in liquid form through the chamber in which the Dry Ice is stored. At its lowermost portion, the cooling unit is connected as by a tube 17 and line 18 with the lower header 19 of an evaporator unit arranged outside and closely adjacent the enclosure 11.

2,730,871 Patented Jan. 17, 1956 ice The evaporator unit includes a plurality of vertically rising tubes joining a header 20 at their upper ends and these tubes are preferably enlarged in size as they rise upwardly. For example the tubes are shown as made up of three sections; the smallest section 21 being at the lower end, the next larger size 22 being intermediately disposed and the largest size 23 being at the upper end. The reason for this variation in size is, as described in my co-pending application hereinabove referred to, to take advantage of the refrigerating effect of expanding fluids. Fins, as indicated at 24, may be used on some or all of the tubes to enhance heat transfer in the usual manner.

The upper header 20 of the evaporator unit is con nected by suitable conduits with a condenser in the upper portion of the enclosure 11. This condenser and the connections thereto will presently be described in detail.

In practice, the secondary refrigerant is a highly volatile liquid preferably non-inflammable and non-toxic such as the liquid known commercially as Freon. This refrigerant in liquid phase passes through the ice chamber by way of the cooling tubes therein to be cooled to a very low temperature due to the sublimation of the solid carbon-dioxide in this chamber. As the cooled liquid flows downwardly through the tube 18, it enters the evaporator and absorbs sufficient heat from the atmosphere in the space to be cooled to convert it to its gaseous or vapor phase in which condition it moves upwardly, thence through the condenser and back through the cooling unit.

A horizontal partition 25 and a baffle 26 are disposed in the enclosure 11 to enable the cold vapor of sublimation from the carbon-dioxide to pass into the condenser chamber from which it is exhausted as by a conduit 27 to the interior of the insulated walls 101 of the cargo space where it serves to enhance the insulating quality of these walls. A fan, such as indicated at 28, circulates air from the space to be cooled over the evaporator which is enclosed by a bafiie wall 29 to direct the fiow of air so circulated. The air enters the lower portion of the enclosure formed by the Wall 29 and is drawn over the evaporator coils and exhausted in front of the fan. The fan may be electrically powered if such power is convenient or, as in the case of a truck or railway car, it may be driven by a suitable mechanism connecting it with the wheels of the car for operation while the car is in motion.

A dehumidifying agent such as a pad of silica gel, indicated at 30, is preferably employed to intercept and dehumidify the air before it reaches the evaporator coils to prevent the accumulation of frost thereon. Any condensation in the vicinity of these coils may be drained through a tube 31 which connects with the lower portion of the batfie wall 29. Circulation of the secondary refrigerant may be stopped by a thermostatically actuated valve 32 which is controlled by a thermo-static element 33 arranged in the path of air approaching the evaporator coils. The element 33 is connected with a valve 32 in the usual manner through a tube 34.

The condenser which is disposed in the upper portion of the enclosure 11 is, as best illustrated in Fig. 2, a double system comprising two headers 36 and 37 connected as by separate condenser coils 38 and 39, respectively, with a single header 40 from which condensed secondary refrigerant flows through a line 41 to the uppermost of the headers 1-6 in the cooling unit. The evaporator header 2% is connected to both of the headers 36 and 37. A direct connection is made with the header 36 by means of a line 42, see Fig. 1, and a connection is made with the header 37 as by a line 43 in which is included a small vacuum gas pump 44. The pump 44 is preferably electrically driven from any suitable source and if it is not in operation, the

system will act in accordance with natural thermo-siphonic-principles. Inother Words, the vapors from the evap- "the evaporator and a slightly higher pressure in the condenser tends to'facilitate both evaporation and condensing ,of the secondary refrigerant in the sealed system so as to. incease the rate, of heat exchange and thus' enhance the efliciency of the entire refrigeration system. This is accomplished by the pump 44 which, when in operation, tends to evacuate the header which is above the liquid level in the evaporator unit and discharges pressure into the header 37 of the condenser, Under these conditions, the condenser coil 39 delivers the refrigerant to the lower header 40. Since, during operation of the pump 44, the header 20 is connected by the line 42 with the header 36 and coil 38 with the header 40, there is communication between the liquid level in the evaporator and the liquid level on the opposite. side of the system which would be in the lower portion of the condenser coils.

Consequently any tendency of the pump to raise the liquid level. in the evaporator, so that liquid rather than vapor would be transferred from one side of the system to the other by the pump, is eliminated. This is true line 42 and the coil 38 of the condenser'unit so that it isexerted upon the liquid on both sides of the system and -a balanced condition results.

The pump pressure is delivered to a relatively large header 37 which is in communication with a large number of condenser coils. It is apparent that some of this pressure may escape back toward the header 20 through the pipe 42. This pipe, however, is as shown in the drawings, small compared to the size of the header 37 and since the pump is in continuous operation, it need only supply pressure to the header slightly faster than it is possible for this pressure to be transmitted back through the small pipe 42. It is' understood, of course, that the variation in pressure need not be great to effect a marked improvement in the otherwise thermo-siphonic process which takes place in the'present refrigeration system. 7

Though the pump 44 may be of very small capacity utilizing very little power, it creates sufiicient vacuum on the evaporator side of the system'and suflicient pressure on the condenser side of the system to greatly enhance the heat transferring properties of the refrigerating system. One ofthe advantageous'results of ajs'ystem of high refrigerating potential such as provided by the present invention is that it enables the cargo of trucks, railroad cars or the like to be rapidly cooled in transit thus obviating the necessity of pre-cooling from outside refrigerating sources prior to placing merchandise in transit. Furthermore if power for the pump 44 or fan 28 or both is obtained through motion of the vehicle, refrigeration will still take place when the vehicle is at rest because the system operates with a natural thermosiphonic cycle when the pumpis not in operation.

I claim:

1. In a refrigeration system which comprises an enclosure for a primary refrigerant and a sealed continuous circuit for a secondary refrigerant including a condenser and a cooling unit utilizing said primaryrefrigerant within the enclosure and an evaporating unit outside the enclosure, the improvement which comprises two separate sets of coils forming the condenser, separate connections between the evaporating unit and each set of coils, and pump means in one of said 'connections to draw fluid from the evaporator and discharge toward the cooling unlt.

2. In a refrigeration system which comprises a closed circuit for a secondary refrigerant including a cooling unit, an evaporator unit, and a condenser unit in which the condenser 'unit isdisposed at a higher level than the other units and forms a connection between them, a pump to draw fluid from the upper portion of the evaporator unit and to direct it through the condenser toward the cooling unit, and a separate line connecting the upper portion of the evaporator unit and the upper portion of the cooling unit to prevent the pump from raising the liquid level in the evaporator.

3. In a refrigerating system which comprises a closed circuit for a volatile fluid secondary refrigerant including a cooling unit, an evaporator .unit, and a condenser unit in which the condenser unit is disposed at a higher level than the other units, two separate coils in the condenser unit both connecting with the coolingunit, a separate connection between each of said coils and the upper portion of the evaporator unit, and a pump in one of said connections to pump fluid from. the evaporator to the cooler through one of said coils, whereby when the pump is out of operation the fluid can circulate through the other connection and the other of saidcoils;

References Cited in the file of this patent UNITED. STATES PATENTS 

